The present invention relates to a chain transmission device for controlling the timing system of an internal combustion engine with two overhead camshafts.
Single-chain control of both the camshafts of internal combustion engines of the above type poses serious problems in terms of size.
That is, the distance between the axes of the camshafts is a design feature which depends on the valve arrangement, and, for reasons of size, should be as small as possible.
On the other hand, the drive shaft the camshafts have a specific transmission ratio (1:2), which means, assuming both camshafts are controlled by a single chain controlled by a sprocket on the drive shaft, the diameter of the driven sprockets on the camshafts is twice the diameter of the drive shaft sprocket.
As the pitch of the chain is substantially imposed by the power transmitted, and the number of teeth, and hence the diameter, of the drive shaft sprocket cannot be reduced beyond a given limit, the resulting diameter of the driven sprockets is fairly large and incompatible with the design distance between the camshaft axes.
In some known engines, the problem has been partially solved using two chain transmissions in series with each other: a first transmission from the drive shaft to an idle intermediate shaft, and a second transmission from the intermediate shaft to the two camshafts. Such a solution, however, is relatively complex and expensive, by calling for an additional intermediate shaft, which serves no other purpose, and for respective supporting means on the engine.
It is an object of the present invention to provide a chain transmission device for an internal combustion engine with two overhead camshafts, designed to overcome the aforementioned drawbacks in a straightforward, effective, low-cost manner.
According to the present invention, there is provided a chain transmission device for controlling the timing system of an internal combustion engine with two overhead camshafts, and comprising a first chain transmission from the drive shaft to an intermediate shaft, and a second chain transmission from the intermediate shaft to both of the camshafts; wherein the first transmission defines a step-down transmission ratio between the drive shaft and the intermediate shaft; and the second transmission defines a step-down transmission ratio between the intermediate shaft and the camshafts; characterized in that the intermediate shaft is the drive shaft of a fuel pump of said engine; and in that the chain of the second transmission has a pitch smaller than the pitch of the chain of the first transmission.
A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawing.